Refrigerating or heat pump system with heat rejection at supercritical pressure

ABSTRACT

A refrigerating or heat pump system includes an evaporator ( 23 ), a compressor ( 20 ), an air-cooled heat rejecting heat exchanger ( 21 ) and an expansion device ( 22 ) being connected in a closed circuit and operating in a trans critical vapor compression cycle. The heat rejecting heat exchanger ( 21 ) is cooled by natural upwards circulation/convention of air. In a preferred embodiment the heat rejecting exchanger ( 21 ) is built into an air flow conduit or shell ( 11 ) to improve the natural air circulation by obtaining a chimney effect.

FIELD OF THE INVENTION

[0001] The present invention relates to refrigerating or heat pump systems, in particular to refrigerating systems for retail and/or storage cabinets for cooling or freezing of food or beverages, or heat pumps for building heating, in both cases using carbon dioxide as the refrigerant.

DESCRIPTION OF PRIOR ART

[0002] Refrigerating systems for cooling or freezing cabinets usually have a refrigerant that operates in a vapour compression cycle with evaporation and condensation. The refrigerant is chosen so that its critical temperature is well below the required heat rejection (condensing) temperature. In order to achieve effective condensation in air-cooled systems, a relatively high air flow rate is required, with large space requirements for the condenser and the air flow system. A fan is needed in most systems to circulate air over the condenser. One problem with this solution is the relatively large power requirement for the fan, and the additional space requirements for the fan and its air flow system. The forced air flow and the fan and its motor may also result in noise problems, and the installation of a fan gives added cost and complexity to the system.

[0003] Residential and light commercial heat pumps that supply heat to the indoor air usually have an indoor unit with forced air circulation over the condenser. Again, an air circulation fan or blower is needed, giving additional power consumption and noise. Furthermore, the thermal comfort may be compromised due to draft from large air flow rates and/or high-velocity air currents with temperature only slightly above room temperature. Due to the large air flow requirements, the indoor unit design needs a large volume, which reduces the options for attractive product design

[0004] Refrigerants in present refrigerating or heat pump systems are either fluorocarbon-based chemicals that are undesirable due to ozone-depleting properties and/or their contribution to man-made climate change, or they are flammable hydrocarbon-based fluids that are questioned due to safety concerns.

[0005] In a trans critical system, heat is rejected by reducing the temperature of the super critically pressurized refrigerant, and not by condensation at constant temperature as in conventional systems. As the supercritical-pressure refrigerant flows through the heat exchanger, it gives off heat and its temperature is reduced (gliding temperature). Ideally, the refrigerant temperature will approach the air inlet temperature, with counter current refrigerant and air flow.

[0006] In a situation with gliding temperature heat rejection from the refrigerant, the air flow rate may be reduced and the air outlet temperature increased compared to the situation in a condenser. In a condenser, the air outlet temperature necessarily has to be below the condensing temperature. In a trans critical system, the high air temperature and reduced air flow rate will be beneficial for natural convection air flow over the heat exchanger, it will reduce noise, and will also be advantageous with respect to thermal comfort in heat pump applications.

SUMMARY OF THE INVENTION

[0007] In view of the above problems and shortcomings it is therefor an object of the present invention to provide a refrigerating system that uses a safe and environmentally friendly refrigerant in a system with a compact natural air circulation heat rejection system without fan power requirements, or with only minor fan power in high load situations.

[0008] To achieve these objects, the present invention describes a system using the nonflammable, nontoxic and environmentally friendly fluid carbon dioxide (CO₂) as the refrigerant.

[0009] The invention is characterized in that the refrigerant rejects heat at a supercritical pressure with gliding temperature through a heat rejecting heat exchanger which is cooled by natural upwards circulation/convection of air as defined in the attached independent claim 1.

[0010] Preferred embodiments of the invention are further defined in the dependent claims 2-8.

[0011] By taking advantage of the special thermodynamic properties of CO₂ and by properly designing the system, heat rejection may, as stated above, take place with natural convection flow of the air, with greatly reduced air flow rate and without the need for a special air circulation fan.

[0012] The invention will be further described in the following by way of example and with reference to the drawings where:

[0013]FIG. 1 shows a trans critical vapor compression system including a compressor, an air-cooled heat rejecting unit with natural air circulation, an expansion device and an evaporator connected in a closed circuit.

[0014]FIG. 2 shows a cross-sectional view of a heat rejecting unit with natural air circulation including an air flow conduit and a heat rejecting heat exchanger based on round tubes in an in line layout according to the invention.

[0015]FIG. 3 shows a cross-sectional view of a heat rejecting unit with natural air circulation including an air flow conduit and a heat rejecting heat exchanger based on round tubes in a staggered layout according to a second embodiment of the invention.

[0016]FIG. 4 shows a side view of a heat rejecting unit with natural air circulation having an air flow conduit and a heat rejecting heat exchanger based on folded tubes according to a third embodiment of the invention.

[0017]FIG. 5 shows a cross-sectional view of a heat rejecting unit with an air flow conduit and a heat rejecting heat exchanger formed into a spiral geometry according to a fourth embodiment of the invention,.

[0018]FIG. 6 shows a heat rejecting unit where the tubes are attached to a plate to increase the air-side heat transfer surface according to a fifth embodiment of the invention.

[0019]FIG. 7 shows a fully counter-flow heat rejecting unit with natural air circulation using Multi Port Extruded (MPE) heat exchanger with plate fin extended surface on one or both side of the said heat exchanger.

[0020]FIG. 8 shows example of the embodiment according to claim 5 used in a refrigerator or similar devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] With reference to FIGS. 1 to 6, the embodiments of the invention will be explained in detail in the following text.

[0022]FIG. 1 shows an example of a vapor compression system including a compressor 20, air-cooled heat rejecting unit 21, expansion device 22 and evaporator 23. The components are connected in a closed circuit that operates in a trans critical vapor compression cycle, i.e. with super critical high-side pressure. The heat rejecting heat exchanger 21 is cooled by natural upwards circulation/convection of air.

[0023]FIG. 2 shows a cross-sectional view of a heat rejecting unit with an air flow conduit or outer air flow shell or jacket 11 and heat exchanger tubes 10. The tubes are arranged in line above one each another within the shell 11. Air enters at the inlet i in the lower end of the system, and exits at the outlet o at the top. Air circulation is achieved by natural convection when the air is heated by the heat exchanger tubes. High-temperature refrigerant from the compressor enters through the heat exchanger refrigerant inlet 12 and flows through the heat exchanger while rejecting heat to the air whereby an efficient chimney effect is achieved. The cooled refrigerant exits from the heat exchanger through the outlet 13. In order to further increase the air flow rate, an extra vertical length of conduit 11 a may be added above the heat exchanger, to increase the chimney effect. The “chimney” or stack may also be built with a converging and diverging nozzle cross section, in order to improve air flow.

[0024] As shown by the cross-sectional view in FIG. 3, the heat transfer tubes 10 may also be arranged in a staggered fashion inside the flow conduit, to increase the surface and improve the heat transfer.

[0025]FIG. 4 shows a side view of a natural air circulation heat rejecting unit with air flow conduit 11 and a heat exchanger based on folded tubes 10. In order to maximize air circulation and heat exchanger efficiency, the refrigerant should flow in a generally counter current direction to the air. With refrigerant inlet at the top 12 and outlet at the bottom, 13 as indicated in the figure the desired relationship between the two different air and refrigerant flows is achieved.

[0026] Another possible embodiment is shown in FIG. 5, where the air flow conduit 11 has a circular cross-section, and the heat transfer tube 10 is formed into a spiral inside the air flow conduit 11. In order to optimize the cross-section of the air conduit 11 with respect to air flow, an annulus containing the heat transfer tube may be established by inserting an inner circular tube into the conduit, the inserted tube being closed at the ends.

[0027] As shown by FIG. 6, the heat transfer tube may form an integral part of the shell in a plate or a conduit 11, i.e. being built into the conduit or shell, in order to increase the heat transfer surface facing the air flow. If necessary, thermal conduction along the height of the conduit can be reduced or eliminated by having slots, splits or louvers 14 in the plate.

[0028] The shell plate or conduit may have a flat surface, or the surface may consist of vertical fins or open or closed duct-like structures that improve natural-convection air flow.

[0029] The invention as defined in the attached claims is not limited to the examples as shown in the figures and explained above, thus in all the above embodiments, one or several walls of the conduit or shell may be applied as heat transfer surface as well. Further, even though the heat transfer tube is shown with a circular cross section in the diagrams, any tube geometry may be used, including flat tubes, oval tubes, multi port tubes and more complex geometry. Still further, the refrigerant tube may also be integrated into the air flow conduit material, giving an integral heat rejecting and air conduit unit which can also enhance heat transfer by radiation. Several enhancements and exterior surface extensions are also possible for the heat transfer tube, including wires, fins, studs etc. An example is shown in FIG. 7 using Multi Port Extruded (MPE) heat exchanger with plate fin extended surface where high temperature refrigerant enters at the top and leaves from the bottom after being cooled by natural upwards circulation/convection of air in fully counter-flow heat exchange process which is ideal in such cases.

[0030]FIG. 8 shows example of the embodiment according to claim 5 used in a refrigerator or similar devices. The heat exchanger 10 is placed in the bottom compartment, with the air flow conduit 11 a behind the refrigerator, extending the air flow shell or jacket 11 in order to enhance the natural air flow/circulation. 

1. A refrigerating or heat pump system including at least one of each following components, an evaporator (23), a compressor (20), an air-cooled heat rejecting heat exchanger (21) and an expansion device (22), connected in a closed circuit operating in a vapor compression cycle, characterized in that the refrigerant rejects heat at a supercritical pressure with gliding temperature through the heat rejecting heat exchanger (21) which is cooled by natural upwards circulation/convection of air.
 2. A system according to claim 1, characterized in that the heat rejecting exchanger (21) is built into an air flow conduit or shell (11) to improve the natural air circulation.
 3. A system according to claim 1 and 2, characterized in that the air flow conduit (11) extends upwards in a generally vertical direction
 4. A system according to the preceding claims 1-3, characterized in that the flow of refrigerant through refrigerant pipes or conduits (10) of the heat rejecting heat exchanger (21) passes is generally from an inlet (12) at the top towards an outlet (13) at the bottom.
 5. A system according to one or more of the preceding claims 1-4, characterized in that an additional air flow conduit (11 a) is installed on top of the heat rejecting heat exchanger (21) in order to increase the air flow rate.
 6. A system according to one or more of the preceding claims 1-5, characterized in that an additional air flow nozzle arrangement is installed on top of the heat rejecting heat exchanger (21) in order to increase the air flow rate.
 7. A system according to one or more of the preceding claims 1-6, characterized in that the air flow conduit or shell (11) is fully or partly used as heat transfer surface whereby the refrigerant pipes or conduits (10) form an integral part of the conduit or shell (11).
 8. A system according to one or more of the preceding claims 1-7, characterized in that a fan is installed before, after or inside the air flow conduit (11) in order to improve air flow at high-load situations.
 9. A system according to one or more of the preceding claims 1-8, characterized in that carbon dioxide is used as irigerant. 